Molecular-dynamics simulation of thin-film growth and relaxation

We simulate the growth of a thin film in two dimensions with a computer implementation of the molecular dynamics (MD) method. The system consists of a krypton substrate maintained at a temperature of about 10 degrees Kelvin, toward which argon atoms are periodically directed (with a velocity corresponding to 120 degrees Kelvin). The resulting argon film follows the (horizontal) spacing of the krypton lattice until the thickness of the film approaches an average thickness of about 10 monolayers. As deposition proceeds, the configuration of the film changes to incorporate an edge misfit dislocation at the film-substrate interface; this relieves the interfacial stress. We also apply the MD method to study the relaxation of thin-film structures predicted by a hard-disk growth model. We consider two variations of the growth model; the first is similar to that described by Henderson et al., the second is a variation which incorporates the effect of surface diffusion. The voids in the relatively open microstructure predicted by the Henderson model are very effective in relieving interfacial stress. The numerous lattice defects (grain boundaries, dislocations, and vacancies) in the denser microstructure predicted by the second type of hard-disk model result in a film with high stress.